1. Field of the Invention
This invention relates to multicelled gas discharge display/memory devices and more particularly to methods of and apparatus for operating such devices with preferred waveforms for addressng and sustaining functions which tend to optimize the performance of the devices.
2. Description of the Prior Art
Multicelled gas discharge devices as display and/or memory units have been proposed in the form of a pair of opposed dielectric charge storage members which are backed by electrodes, the electrodes being so formed and oriented with respect to an ionizable gaseous medium as to define a plurality of discrete gas discharge cells. Charged particles (electrons and ions) produced upon ionization of the gas volume of a selected discharge cell, when proper alternating operating voltages are applied between opposed electrodes, are collected upon the surface of the dielectric at specifically defined locations and constitute an electrical field opposing the electrical field which created them. Those collected charges aid an applied voltage of the polarity opposite that which created them so that they aid in the initiation of another discharge by imposing a total voltage across the gas sufficient to again initiate a discharge and a collection of charges. This repetitive and alternating charge collection and ionization discharge constitutes an electrical memory of a cell in the on state of discharge. With properly chosen values of the alternating voltage, cells in the off state of discharge remain in that state during the alternations hence that state is also retained in electrical memory.
The alternating voltage offering the above memory characteristics is termed a sustaining voltage. For a given device it usually has a range of values.
Change of the state of individual cells in a device subject to a sustaining voltage has been accomplished by superimposing voltage pulses on the sustaining voltage. Cells in an off state of discharge have been turned on by pulses, usually applied to the opposed electrodes of the selected cell, which raise the voltage imposed across the gas to a level which initiates an ionization discharge of a magnitude to cause sufficient charged particles to collect on the dielectric surface of the cell to cause a repetition of the discharge by virtue of the augmentation of the reversed sustainer voltage with the wall charge voltage. Cells in the on state of discharge are selectively manipulated to the off state by applying a voltage pulse across the selected cell in opposition to the currently applied sustainer and of a magnitude sufficient to discharge the wall charge without developing an opposite wall charge at the on state level. In each of a turn on discharge and a turn off discharge a burst of light is emitted over a very short portion of the sustainer half period. For example, where the sustainer is applied at a typical 50 kilohertz the light burst of on state cells may be of about 500 nanoseconds in the initial transition portion of each ten microsecond half cycle were essentially a square waveform is imposed.
Sustainer voltage waveforms having regular periods are conventionally developed in various forms. One typical prior art form involved an essentially square wave developed from two components by applying a sustainer voltage level V.sub.s to one array of electrodes of a pair of opposed arrays making up the matrix of cells of the device for an interval which is less than half a period. An addressing pedestal was then imposed for an interval which typically could extend up to the balance of the half period at a level intermediate the sustainer voltage V.sub.s and the reference voltage level. During the second half of the sustainer period the first sustainer component remains at its reference level and the second sustainer component shifts from its reference level to V.sub.s for a suitable interval after which it shifts to a pedestal of a suitable addressing level. The first and second components, if considered as x and y components in a cell matrix made up of an x array of electrodes transversely oriented to a y array of electrodes, imposes a composite sustainer waveform, x-y, which typically is made up of a step from the reference level to positive V.sub.s, a step from positive V.sub.s to V.sub.M and a step from V.sub.M to the reference level V.sub.N for the first half period, then a step to negative V.sub.s, a step from negative V.sub.s to V.sub.M negative, and a step from V.sub.M to the reference level V.sub.N for the second half period.
During the transitions to the V.sub.s levels, cells in the on state of discharge are discharged. The displacement of charged particles in the ionized gas of the cells and the accumulation of those particles to develop a neutralizing wall voltage on the dielectric separating the gas from the electrode or electrodes of the cell requires a time interval which imposes a lower limit on the interval V.sub.s is imposed. A further time limit is imposed for cell manipulation. Thus, where a cell is addressed for writing by application of a write signal a sufficient interval is allowed prior to a transition to the opposite polarity of V.sub.s for the discharge of the cell from an off state to an on state to stabilize its wall voltage at or near the neutralizing level of an on state cell. When a cell is erased, its transfer to the off state from the on state is by application of an erase signal which discharges it from its on state wall charge to a wall voltage at or near its "neutral" wall voltage whereby subsequent sustainer transitions are not augmented by any residual wall voltage sufficiently to ignite a further discharge, again requiring a given amount of time prior to further transitions. These time intervals have been found to be dependent on a number of physical parameters of the cells and thus of the devices made up of a matrix of cells including gas volume thickness, dielectric interlayer thickness, gas composition and pressure and applied operating potentials. While a range of operating parameters are operative, these parameters are chosen to place the widest range of cell dimensions within the operative range whereby an acceptable yield of multicelled devices is achieved. Further, the cycle waveform and period has been regular and has been chosen as a compromise to realize commercially acceptable device yields where all cells are operative for manipulation of their discharge states when addressed. Such traditional regular waveforms limit device performance by not optimally using the time available and by forcing compromises on operating tolerances, current requirements and brightness and contrast levels.
An object of the present invention is to optimize the dynamic waveforms applied to multicelled gas dischare display/memory devices.
Another object is to enhance operating margins for multicelled gas discharge display/memory devices.
A further object is to improve device performance and yield for multicelled gas discharge display/memory devices. More particularly objects involve achieving satisfactory operation with slower gas mixtures whereby brightness of discharge displays is increased, and lower currents and thus reduced dielectric thicknesses are tolerable. Additional performance improvements include reduction in the static firing voltage of cells and increase in the tolerable geometric non-uniformities of discharge devices.
A fifth object is to achieve more reliable operation of multicelled gas discharge display/memory devices with respect to the selective manipulation of the charge state of individual cells.